a second letter to president obama as well. this as we're getting word of a possible person of interest in the case. just a little background on this poison, ricin is a poison that is found naturally in caster beans, and any amount, an amount in fact the size of a head of a pin can kill an adult. there is no specific test for exposure and there is no antidote once exposed. there have been two cases recently the fbi discovered several vials of ricin in a motel room near the vegas strip in 2008 and in 2004 authorities discovered traces of the poison on a letter opening machine in the office of then senate majority leader bill frist. what have we learned from this fbi bulletin. >> reporter: the secret service is acknowledge thag a letter addressed to press was received at a remote white house mail screening facility, the emphasis being that it never got close to the white house complex or obviously the president himself. as for that fbi operations bill tin we've obtained, here is some of what it says. that there were two letters o